Thickened aqueous fluids, such as drilling fluids, are widely used in oil field operations for fracturing and for well servicing, such as drilling, workover and completion. The aqueous fluids typically have high viscosities and are able to minimize solvent loss. They often contain a hydrophilic polymer dissolved in a solvent, such as water. The water-soluble polymers most often used are polysaccharides, guar and guar derivatives, cellulosics and cellulosic derivatives, xanthan gum, modified starches, polyvinylsaccharides, and similar biopolymers. The hydrophilic polymers act to thicken the fluids; crosslinking agents and additives such as surfactants may also be used. The polymer can be offered as a powder or as a suspension in a carrier fluid such as No. 2 diesel oil. Polymers in powder form must first be dispersed so that individual particles can absorb water in order to prevent the formation of lumps. As a result, polymers in suspensions have been developed for delivery to work sites, and they are usually preferred over polymers in powder form. Such suspensions tend to disperse well when added to water.
The carrier fluid used to suspend the polymer is an important choice. A typical concentrated suspension uses guar or guar derivatives dispersed in a hydrophobic solvent, such as No. 2 diesel, in combination with a suspension agent and a surfactant, such as that described in U.S. Pat. No. 6,800,593 to Dobson, Jr. et al. However, such oil-based drilling fluids are not biodegradable and have fallen into disfavor due to environmental concerns.
Recent patents that describe alternative carrier fluids include U.S. Pat. No. 6,620,769 to Juppe et al., which uses white mineral oil as the carrier fluid; U.S. Pat. No. 6,667,354 to Fox et al., which uses carboxylic acid ester as the carrier fluid; and U.S. Pat. No. 7,199,084 to Parris, which uses a “biodiesel” (any of the mono alkyl esters of long chain fatty acids) or a “mutual solvent” (such as ethylene glycol monobutyl ether and hexylene glycol).
The choice of a carrier fluid involves a variety of issues, including cost of the chemicals, toxicity, viscosity of the suspension, cross-linking considerations, and, most importantly, the stability of the suspension. Ideally, a carrier fluid will aid in the production of a polymer suspension that is water soluble, environmentally acceptable, cost effective, low in effective viscosity, and stable for long periods of time to allow for storage and transportation of the suspension to the field.